Titanium and titanium alloy are generally provided in the form of ingots obtained from sponge titanium or titanium scrap materials by melting using the consumable electrode type vacuum arc melting method or the electron beam melting method and solidification. These ingots are subjected to blooming, forging, rolling or other hot-working, and are worked to the shape of slabs which can be rolled using a hot rolling mill, and then are touched up on their surfaces to obtain slabs for hot rolling.
In the melting process, the consumable electrode type vacuum arc melting method is being widely used, but the arc discharge between the electrode and the casting mold has to be uniformly performed, so the casting mold is limited in shape to a cylindrical mold. As opposed to this, in the case of the electron beam melting method using a hearth or the plasma arc melting method, the melt of the titanium which was melted in the hearth flows into the casting mold, so there is no limit on the casting mold shape. Not only a cylindrically shaped, but also a block shaped ingot can be produced. When using a block shaped ingot to produce a flat product, due to its shape, it is conceivably possible to perform the hot rolling while omitting the blooming, forging, and other hot-working process. The costs can be reduced by that amount attributable to the omission. Therefore, the technique of hot rolling while omitting hot-working by using a block shaped titanium ingot which was cast by means of a rectangular casting mold as is as a titanium slab for hot rolling has been studied. Here, the blooming, forging, or other hot-working process which is performed before hot rolling will be referred to overall as the “breakdown process”.
In this regard, in a titanium slab which is cast by means of electron beam melting or plasma arc melting in a block shaped casting mold, the slab produced industrially has crystal grains of a size of several dozen mm in the structure as cast. Further, commercially pure titanium contains some Fe or other impurity elements. Depending on the case, β phases will sometimes form at the hot rolling temperature. The β phases which are formed from the coarse α phases become coarse. The β phases and α phases greatly differ in deformation ability even at a high temperature, so the deformation becomes uneven between the coarse β phases and α phases and large surface defects sometimes result. To remove the surface defects which are formed during the hot rolling, it is necessary to increase the ablation amount of the hot rolled plate surface in the acid pickling process, so the yield deteriorates. That is, as explained above, with a block shaped titanium slab which is produced by electron beam melting or plasma arc melting and for which breakdown process can be omitted, a drop in the production cost can be expected, but a drop in yield would be a concern.
PLT 1 discloses a method of producing a thick plate or slab of titanium during which surface defects are prevented by the method of heating to the (β transformation point+50° C.) or more at the stage of the cast ingot before hot-working, then cooling to a temperature of the (β transformation point-50° C.) or less and refining the coarse crystal grain structure of the cast ingot. However, in PLT 1, the cast ingot is assumed to be a columnar shape. To render it to a slab shape, there is an extremely great drop in yield. Further, the breakdown process before hot rolling is also essential, so compared with a block shaped titanium ingot, the production costs become higher. In addition, a consumable electrode type vacuum arc melting furnace which produces a columnar shaped cast ingot structurally cannot continuously perform the above heat treatment at the time of melting. A heat treatment step is added, so a rise in the production cost is a concern.
PLT 2 discloses a method of drawing out a titanium slab which was smelted in an electron beam melting furnace directly from the casting mold wherein, at the cross-sectional structure of the slab, when the angle θ between the solidification direction from the surface layer toward the inside and the casting direction of the slab is 45° to 90° or the angle between the c-axis of the hcp and the direction normal to the slab surface layer in the crystal orientation distribution at the surface layer is 35° to 90°, the cast skin is good and the surface defects formed by hot rolling are suppressed and a process for hot-working an ingot, such as blooming, forging, rolling or the like, that is, the so-called breakdown process may be omitted. That is, by controlling the shape or crystal orientation of the crystal grains at the surface, it is possible to suppress the formation of flaws due to such coarse crystal grains. However, PLT 2 does not consider the possibility of a large amount of the β phases being formed at the time of heating in the hot rolling. It is believed that good surface properties are obtained, but variations in the operating conditions and the method of slab production are liable to cause the possibility of deterioration of the surface properties.
PLT 3 discloses a method of directly hot rolling an ingot of a titanium material while omitting blooming process comprising melting and resolidifying the surface layer at the surface corresponding to the rolling surface of the ingot by high frequency induction heating, arc heating, plasma heating, electron beam heating, laser heating, etc. to refine the particle size down to a depth of 1 mm or more from the surface layer and improving the structure of the surface layer after hot rolling. This rapidly solidifies the surface layer part to form a fine solidified structure with irregular orientation and thereby prevent the formation of surface defects. As methods for making the structure of the surface layer of the titanium slab melt, high frequency induction heating, arc heating, plasma heating, electron beam heating, and laser heating may be mentioned. However, with the TIG welding method of arc heating which is industrially used for titanium materials, much time is taken for treatment per area. Further, even with a melting method other than arc heating, high expenses are incurred in introduction of facilities for improvement of the structure of the surface layer of the slab. Furthermore, electron beam heating etc. usually have to be performed in a 10−5 Torr or so vacuum, so there are great restrictions facility wise. That is, a rise in the production costs is feared.